As discussed in Human Factors and Unsafe Actions, in the aviation industry, approximately 40% of errors are classified as skill-based errors. Skill-based errors occur when a complete procedure or checklist is available to follow, but errors occur in execution. According to the HFACS classification, the root causes of errors despite having procedures or checklists can be broadly categorized into three types: errors arising from attention, errors arising from memory, and Skill errors. Errors due to attention are called slips, while those due to working memory are called lapses. Skill errors pertain to training issues, which we will not discuss here.
We will mainly introduce general theories of attention and memory in cognitive psychology and point out what happens in the psychological processes of attention and memory that ultimately lead to errors.
Errors Due to Attention
Recall how we use attention resources when performing actions. For instance, planning an extended trip, arranging the main schedule, finding transportation and accommodation, estimating stay duration, and organizing this information all require substantial attention resources. However, some actions, like riding a bicycle, driving, or folding clothes, become effortless tasks barely requiring attention after frequent practice, as if the body moves automatically.
System 1 and System 2
In “Thinking, Fast and Slow,” human thinking is divided into two systems: System 1 and System 2. System 1 is an automatic process, a fast and intuitive way of thinking that relies on experience and intuition for quick judgments. System 2, on the other hand, is a slow and effortful way of thinking that requires more attention and energy for analyzing complex problems or making significant decisions. We all have both systems and use them at different times: System 1 often occurs in familiar, well-practiced daily life, while System 2 intervenes when deeper consideration is needed. The table below may quickly summarize the differences between System 1 and System 2:
| Characteristic | Automatic Process (System 1) | Control Process (System 2) |
| Level of Attention Involvement | Minimal or no attention | High level of attention required |
| Level of Consciousness Awareness | Low awareness | High awareness |
| Use of Attention Resources | Low | High |
| Type of Information Processing | Parallel, simultaneous | Sequential |
| Speed of Information Processing | Fast | Slow |
| Novelty of the Task | Familiar and stable tasks | Unfamiliar tasks, highly variable |
| Depth of Information Processing | Low cognitive processing | High cognitive processing |
| Difficulty of Task | Simple | Difficult |
Source of Errors: When System 1 is Interrupted
However, we can experience the following:
- Forgetting which part of your body you’ve already washed midway through a shower.
- Habitually placing your keys by the door, but after buying frozen foods at the supermarket, putting them in the fridge upon returning home.
This characteristic of System 1 is that due to its automaticity and minimal effort, when familiar actions are abruptly interrupted, the excessive automation becomes the source of errors.
Aviation Practice of Attention Errors: Go-Around Procedures
There was a time when civil aviation agencies would post phrases like “Follow procedures, avoid shortcuts, remind each other, take action early, divert safely, go-around well, cooperate seamlessly to ensure aviation safety” as safety codes in cockpits to remind crew members that any unstable approach condition during approach phases would automatically trigger policy to encourage go-arounds.
For passengers, a go-around during what seems like a normal descent, where the ground is nearly in sight, often results in a scary unexpected maneuver. However, from an aviation operational perspective, a go-around is a standard procedure thoroughly trained and tested for crew members. Unfortunately, in practice, errors in go-around procedures are not uncommon, especially those that could lead to significantly undesirable aircraft states (UAS).
Go-Around Procedure: An Example Using Airbus Aircraft
Using the Airbus Safety First article from July 2011, “The Go Around Procedure,” as an example, let’s look at how to execute a go-around procedure on an Airbus aircraft. When the Pilot Flying (PF) calls out “Go Around…Flaps,” they should:
- Move the thrust levers forward to the TOGA position
- If manually flying during the go-around, pitch the aircraft to the target angle
- Verify the Flight Mode Annunciation (FMA) status
The Pilot Monitoring (PM), upon hearing the PF call “Go Around…Flaps,” should:
- Retract FLAPS one step
- Raise the landing gear after confirming positive climb when the PF calls “Gear Up”
- Monitor PF’s flight operations
When ensuring flight quality, we check that the following steps are executed in correct order while verifying compliance with procedures:
- Step 1: Push the thrust levers to the TOGA position
- Step 2: Retract FLAPS one step
- Step 3: Retract landing gear
The most critical part is moving the thrust levers to the TOGA position, which also:
- Cancels the Approach Mode
- Notifies the FMS about the go-around phase
- Switches the aircraft state from approach to go-around
Thus, pushing the thrust levers to TOGA is essential. When crew decides to go around but the thrust levers are not moved to TOGA, leaving Approach Mode active, it often results in unexpected aircraft behavior, requiring more effort at low altitude with low thrust to handle undesired attitudes. Airbus Safety First has provided examples of severe consequences when go-around procedures are not followed.
Abnormal Cases in Go-Around Procedures
Case 1
On a foggy morning with low clouds (visibility 3000 meters, broken clouds at 200 feet, scattered clouds at 300 feet), the crew of an airline flight conducted an ILS approach manually. When descending past the decision height (200 feet AGL), the crew couldn’t see the runway. At 170 feet AGL, the PF executed a go-around, taking about five seconds to push the thrust levers forward, ultimately stopping at the FLX/MCT detent (1). He pitched the aircraft to about 6 degrees nose up, stopping descent at 150 feet radio height. They retracted flaps one step, setting the aircraft configuration to CONF3 (2).
Four seconds after stopping the thrust levers at FLX/MCT, the captain engaged the autopilot and raised the landing gear (3). Due to not selecting TOGA, the aircraft remained in LAND mode; therefore, the autopilot began pitching down to track the decreasing glide slope height. The PF then moved the thrust levers to the CLB position. Descent continued, triggering a ‘SINK RATE’ alert from the EGPWS at 127 feet radio height, with -3.9 degrees nose down pitch. The PF disconnected the autopilot to fly manually, applying nearly maximum aft stick input to regain control. The closest approach to the ground was 76 feet radio height, at 182 knots airspeed with CONF3 and retracted gear.
Note 1: The thrust levers should have been moved to the TOGA position, not FLX/MCT, which is incorrect.
Note 2: Retracting flaps one step is correct.
Note 3: After moving the thrust levers and retracting flaps, the last step is retracting landing gear, which is correct.
Case 2
On a foggy day, the crew conducted an ILS approach with autopilot and auto throttle engaged. At the decision height, the runway was not visible. They decided to go around at 185 feet radio height but pushed the thrust levers to a position below FLX/MCT (1). Three seconds later, they retracted flaps one step to CONF3 (2). At 57 feet radio height, the captain disengaged the autopilot, triggering a ‘DON’T SINK’ EGPWS warning. The lowest height was 38 feet radio height.
Note 1: The thrust levers should have been moved to TOGA, not below FLX/MCT, which is incorrect.
Note 2: Retracting flaps one step is correct.
(Both examples are referenced from Airbus Safety First August 2010, “Go Around Handling“ 2.1)
Why is This an Attention-Related Error?
Even though there are standard operating procedures for go-arounds and crew have practiced them many times, the first step mistake of not moving thrust levers to TOGA is common. This omission, especially near the decision height, can lead to a substantial undesired aircraft state with rapidly increasing airspeed and continuous altitude loss. In summary, the root causes of this error are:
- The erroneous action: Inadequately pushing thrust levers to TOGA during a go-around.
- The fundamental error: Attention issue; due to over-automation, lack of attention resource involvement leads to systematic procedural errors (System 1).
Memory Errors
Errors stemming from memory faults result in “retrieval failures,” commonly known as “forgetting.” We all experience forgetting—such as forgetting clothes in the washer, leaving the stove on, forgetting to pay a phone bill, and so on. The essence of forgetting is a failure in “retrieval,” but the entire memory process includes both “encoding” and “retrieval,” with failures possible in both phases. Memory exists as three types: Sensory Memory, Short-Term Memory, and Long-Term Memory.
Sensory Memory, Short-Term Memory, and Long-Term Memory
In cognitive psychology, sensory memory is the brief storage of sensory stimuli, lasting only a fraction of seconds, capable of holding a fleeting impression like a glimpse of light or a sound. Short-term memory is the temporary storage space for processing attention-requiring information, typically retaining about 7±2 items for 20-30 seconds. Working memory is an extension of short-term memory, not only storing but also manipulating information for cognitive tasks such as problem-solving, reasoning, and language comprehension. Long-term memory is a largely indefinite storage system that holds information converted from short-term memory, including facts, skills, and experiences. These aspects work in harmony to form the human memory system. Organized table summary below:
| Storage Structure | Encoding | Capacity | Duration | Retrieval | Reasons for Retrieval Failure |
| Sensory Memory | Features of Sensation | 12-20 items | 250ms-4sec | Complete retrieval possible | Masking from latter stimuli; Disappearance |
| Short-Term Memory | Auditory, Visual, Semantic | 7±2 items | 12 sec | Complete retrieval possible | Displacement from new items exceeding capacity and replacing old ones; Interference; Disappearance |
| Long-Term Memory | Semantic, Visual Knowledge; Meaningful Images | Unlimited | Infinite | Proper cues and questions needed, involves recall and recognition | Interference; Inappropriate retrieval cues |
Source of Errors: Too Much, Inappropriate Retrieval Cues, Time Fade
Common error sources in memory include:
- Time-Fade Effect: For example, setting a reminder to turn off the water twenty minutes later but forgetting. This “time-based” task is prone to forgetting without proper reminder cues, often being the source of lapses.
- Too Much Volume: During encoding, having too much to remember ultimately results in forgetting. Research on short-term memory shows an average balance of 7±2, indicating typical capacity is 5 to 9 elements. Nevertheless, individual variations exist, and some, like the absent-minded raccoon, exceed limits after remembering over 3 items. We will later see the impact of encoding volumes on errors in memory cases.
- Inappropriate Retrieval Cues: The difficulty difference between a “multiple-choice question” and an “essay question” exists because “recognition” is easier than “recall.” Therefore, forgetting instructions to buy items but remembering them while wandering around a supermarket employs a “turning essay question into multiple choice” retrieval strategy: going to a supermarket allows you to “see” the item, providing the opportunity to “recall” it.
Aviation Practice of Memory Errors: Omission and Error of Air Traffic Control Instructions
During flights from point A to B, crew members cross multiple Flight Information Regions, served by Air Traffic Controllers (ATC) from different countries. Although ATC clearances use standardized international terminology, variations in intonation by controllers can cause misunderstandings.
Let’s look at an example occurring in the approach phase: after an aircraft is handed off to an airport approach control from the previous sector (like area control), an ATC instruction might be given like this:
- Approach: American 73, good morning, reduced to 180 knots, turn right heading 310, intercept localizer 34 left.
After being given this instruction, the aircraft is required to perform a readback, for instance:
- Response (1): American 73: 180 knots, turn right heading 310, intercept localizer 34 left, American 73.
- Response (2): American 73: turn right heading 310, intercept localizer 34 left, American 73.
The approach instruction contains four items:
- Flight number: Each aircraft has a call sign. In this case, American 73 represents an American Airlines Flight 73.
- Speed: Reduce to 180 knots.
- Heading: Turn right to heading 310.
- Runway or other clearance: In this instance, intercepting localizer for runway 34 left.
Based on crew responses, errors are divided into omissions (what should have been repeated but wasn’t) and mistakes (errors in repetition). In the second response example, the crew omitted the repeat of the “180 knots” instruction, identified as an omission error. Without interaction checks between cockpit crew and ATC, omission and mistake errors may cause crew to not fly according to ATC instructions, leading to ATC violations or, in worst cases, insufficient separation vertically, horizontally, or longitudinally with other aircraft, posing a midair collision risk.
Memory Errors
A series of studies by Molesworth and team (see Wu et al., 2019; Y. H. P. S. A. Y. Dissanayaka et al., 2023) aim to clarify factors affecting memory errors. In one study, the researchers took six sets of 30-minute approach ATC recordings from airports in Sydney (YSSY), Hong Kong (VHHH), Los Angeles (KLAX), and Tokyo (RJTT), with nine ATC interactions each. They analyzed non-native English pilot errors, factors like English fluency and native language background, intonation, speech rate, instruction complexity, and presence of friendly interactions. Some findings were as expected:
- The more information contained in one clearance, the higher the chance of errors. Compared to permissions containing less than three items (average errors 0.96), having more than four greatly increases errors (average errors 1.76).
- Non-clearance-related content does not impact error rate. Wishing good morning, Merry Christmas, or Happy New Year to ATC was neutral to message quality.
- ATC speech rate correlates weakly with error rates.
In summary, the root causes of this error are:
- The erroneous action: Failing to correctly repeat ATC instructions.
- The underlying error: Skill-based error, memory: due to the provided information exceeding the working memory span at the time of encoding, leading to error.
Is There a Correlation Between English Proficiency and ATC Clearance Errors?
However, is there another factor impacting ATC clearance readback quality? Potentially, cockpit crew’s and ATC’s English proficiency. Does having English as a first language result in fewer ATC clearance errors? Unexpected findings emerged:
- In Dissanayaka et al. (2023), English-first-language cockpit crew had higher error rates compared to non-native speakers; furthermore, error types (omissions or mistakes) and content (text or numbers) do not impact this main effect, contradicting prior findings (Wu et al., 2019).
- When English-speaking crew engaged with ATC whose first language was not English, they experienced higher errors. Conversely, with native-English ATC, English-speaking crew made more errors compared to non-native-speaking crew.
Practical Recommendations
Based on these findings, practical recommendations are:
- ATC instructions should ideally not contain more than three items per clearance. When exceeding three items, omission or mistakes increase, leading to incorrect readback. Considering time spent clarifying these errors, it may be more efficient to divide lengthy instructions across two interactions.
- Native-English-speaker advantages are not evident in standardized ATC phraseology communications. ATC communications should be seen as adhering to standard procedures, exhibited through English-based standard terminology. Thus, prediction of communication proficiency should be grounded in procedural adherence rather than English skills.
- If accent provides interference, threat and error management (TEM) during pre-flight briefings could highlight this as a reminder. Feedback emphasizing pronunciation and intonation could potentially aid crew members in mutual understanding and communication clarity.
Conclusion
- Root causes of skill-based errors can be categorized as attention-related or memory-related, with the former identified as slips and the latter as lapses.
- The attention-related slip is an action completion with errors due to disturbances in System 1 or low attention resource allocation, where cognitive automation leads to procedural mistakes.
- Memory-related lapse is overlooking actions due to failures in short-term memory retrieval, resulting in forgetting intended actions.
